This game has extreme framerate drops when 'Extra Details' and (especially!!!) 'Grass' are enabled. I get 60fps most of the time, but there are a lot of places on Myst Island where the framerate drops to 30-45 fps. No matter the resolution. Even on 720p!!! In any case, my GPU utilization never goes higher than 60% (on a GTX 970)

So, it must be of some sort of CPU bottleneck. But the strange thing is, my four CPU cores are nowhere near 100% stressed. More like three of them at 50% and the fourth at 70%. Is this some Unity engine strangeness, or is there something wrong with the Grass setting?

The processor isn't the only thing that will hold a computer back. Big graphics require more memory as well. I don't have a good enough computer to turn half the cool stuff on, but I would guess, barring an actual issue with the game, is that RAM is the culprit here. Both video RAM and system RAM.